The present invention relates to a knitting machine with needle selection device, for manufacturing socks or stockings.
As is known, knitting machines generally comprise a planar or cylindrical needle supporting element in which a plurality of mutually adjacent grooves is defined; a needle, and possibly a sub-needle or selector, are accommodated in each one of said grooves and are slideable longitudinally along their respective groove. The needles or sub-needles are provided with a heel which can protrude from the related grooves of the needle supporting element, on the opposite side with respect to the bottom of the grooves, in order to engage within paths defined by cams which face the needle supporting element. Said paths are constituted by portions which are variously inclined with respect to the longitudinal extension of the grooves so that by actuating the needle supporting element with a transverse movement with respect to the grooves and the cams, a reciprocating movement of the needles along the related grooves causes said needles to grip the thread which is fed to them to form the stitches.
Some types of knitting require the use of all the needles mounted on the needle supporting element, whereas other types of knitting, such as for example the knitting of patterns, require a selection of the needles which must knit at a thread feed.
Various types of device for performing needle selection are known. In particular, in circular knitting machines for manufacturing stockings, i.e. in machines wherein the needle supporting element is constituted by a cylinder which is rotatable about its own axis with respect to the needle actuation cams, selection devices have long been used which use mechanical programmers which, by means of sliders, act on the heels of the sub-needles and are oscillatable in planes which are radial with respect to the cylinder in order to sink the heels in the grooves of the cylinder, thus avoiding engagement of the heels with the subsequent cam, which instead activates the needles having sub-needles which the selection sliders have not acted on.
Said types of device have the disadvantage of not being versatile in use, since in order to vary the selection of the needles it is necessary to intervene manually on the mechanical programmer which, in any case, only allows to produce relatively simple patterns.
Other types of selection device use, instead of mechanical programmers, electronic programmers which act on the heels of the needles or sub-needles by means of electromechanical or piezoelectric actuators. With these devices it is possible to obtain a correct selection even when the needle cylinder is actuated with high rotation rates, and it is possible to change the knitting which can be produced simply by changing the program of the electronic control element which actuates the various electromagnetic or piezoelectric actuators.
These kinds of selection devices, too, are not free from problems. In fact, the use of electromagnetic actuators entails relatively low response speeds which can penalize the productive potential of the machine, whereas piezoelectric actuators, which have higher response speeds, are fragile and expensive to manufacture and control.
Another method, adopted in order to increase the selection speed in combination with the above described methods, consists in differentiating the needles, or rather the sub-needles, into a plurality of rows, so that the heels of adjacent sub-needles are axially offset with respect to one another. This method uses as many actuators as there are rows of sub-needles. Each actuator can use, for its actuation, the time required for the transit, during the rotation of the needle cylinder, of all the sub-needles which belong to other rows, i.e. of all the sub-needles which have heels at different levels with respect to the level of the heel which can be actuated by means of that actuator. With this method it is possible to achieve high machine operating speeds, and thus high productivity, even with actuators which have relatively low response speeds.
However, this method has the problem that it must use very long sub-needles which entail a greater length of the needle cylinder and increase friction in the machine. Furthermore, the fitting of sub-needles divided into various rows is obviously more complicated than mutually identical sub-needles.